As a simple method of adjusting PID parameters for a PID controller, an auto-tuning method based on limit cycles has been proposed (see, for example, patent reference 1). Such a simple method is executed on the assumption that when a controlled system is expressed as a transfer function, the degree or the like of the function takes a specific value. In this method, PID parameters are roughly adjusted.
In order to make finer adjustment than the above simple method, a method of determining PID parameters by analyzing process parameters (a process gain, a process time constant, and the like) including the degree of a controlled system has also been proposed (see, for example, patent reference 2). According to this technique, for example, a modeling step of approximating a characteristic of a controlled system to a transfer function expression, and PID parameters are determined by referring to the modeling results. In this case, traditional adjustment formulas such as the CHR (Chien, Hrones, Reswick) method and IMC (Internal Model Control) method are used.
Note that the present applicant has not found any prior art references associated with the present invention other than the prior art references specified by the prior art reference information described in this specification:    patent reference 1: Japanese Patent Laid-Open No. 2000-155603    patent reference 2: Japanese Patent Laid-Open No. 2002-351502